frontswap: enable call to invalidate area on swapoff

During swapoff the frontswap_map was NULL-ified before calling
frontswap_invalidate_area().  However the frontswap_invalidate_area()
exits early if frontswap_map is NULL.  Invalidate was never called
during swapoff.

This patch moves frontswap_map_set() in swapoff just after calling
frontswap_invalidate_area() so outside of locks (swap_lock and
swap_info_struct->lock).  This shouldn't be a problem as during swapon
the frontswap_map_set() is called also outside of any locks.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Shaohua Li <shli@fusionio.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>